This invention relates to screening devices used to classify particulate material, and specifically relates to a feed prestratification attachment to be mounted upon a conventional vibrating screen unit.
A vibrating screen unit is a device which accomplishes the separation of a source of particulate feed material into various size classes. This classification is accomplished by feeding the material to the unit, which is vibrating in a controlled reciprocating manner in a vertical plane, so that particles which are fed thereto are repeatedly thrown upward, and, upon falling, the finer particles pass through at least one deck of screening surface containing apertures for passage of smaller, undersize, particles. The larger, oversize, particles are retained upon the upper deck of screening surface. The screening surfaces are arranged so that the uppermost deck is provided with the largest screen openings and the lowermost deck is provided with the smallest openings.
A counterweight shaft apparatus induces the vibrating action to the screen unit and provides it with a specified amplitude and frequency. The amplitude induces the upward motion to the layer of feed material deposited upon the uppermost deck of screening surface. This upward motion lifts the feed particles away from the screening surface to allow the layer of material to rearrange itself before falling back upon the screening surface for separation. This rearrangement of particles is known as stratification. Ideally, through the stratification process, the oversize particles move to the top of the layer and the undersize particles move to the bottom to be presented to the screening surface for separation. The oversize particles also act to push the close-to-aperture, near-size, particles through the screening surface. Once separated, the classified particles are separately fed in conventional fashion to conveyors or collecting hoppers.
A common problem of conventional vibratory screen units is that the feed material tends to heap upon the uppermost end of the top screening surface, thus creating a bed of material which is too thick to permit proper stratification. Even the vibration of the screen unit will not permit efficient stratification of that heaped material and the desired presentation of undersize particles to the screening surface for classification.
One attempted solution is to increase the amplitude of the unit so that the heaped particles are thrown higher, allowing more time for better stratification. Unfortunately, as amplitude is increased, frequency has to be decreased. A significant consideration here is that screen units are designed with certain speed and amplitude parameters to achieve an acceptable acceleration force level, Ag.
At lower speeds, the particles will be presented to the screening surface a lesser number of times. This will decrease capacity, as undersize particles are provided with less opportunities to pass through the screening surface. The lower the amount of separation of undersize particles in the feed, the lower is the screening efficiency.
In an attempt to solve this problem of "heaping" of feed material, supplemental feed stratification units known as "grizzlies" have been provided. These grizzlies generally consist of a static or fixed screening surface positioned above the feed end of the screen unit to assist in the stratification of feed material. A common drawback of these static grizzlies is that the heaping of feed material is only transferred from the screen deck to the grizzly itself. Thus, the stratification of the feed material is still not enhanced significantly, and the undersize particles may still be prevented from contacting the screen apparatus by oversize and near-size particles.
Thus, there is a need for a prestratification screening device which allows incoming feed material to be stratified prior to its introduction upon the screening surface. In addition, there is a need for a screen attachment which will allow the conventional screen unit to be operated at optimum speed and amplitude parameters to enhance screening capacity and efficiency without exceeding the designed Ag force level. Another problem associated with the heaping effect is the uneven distribution of the feed material across the width of the screening surface. Therefore, a feed prestratification attachment should also provide a more even distribution of the feed material across the screening surface. Lastly, there is a need for a prestratification screen attachment which may be mounted upon any conventional type of vibrating screen unit.